The present invention relates in general to communication line signal control and, in particular, to a new and useful circuit arrangement for controlling signals to reduce attenuation.
In electrical communications, subscribers' terminals, for example, telephone sets, are connected to subscriber lines which vary in length depending on the remoteness of the telephone exchange. This results in unequal subscriber line losses or unequal input impedances of the line, whereby a stronger attenuation of the sidetone signals in the hybrid circuit incorporated in the subscribers' terminal is made more difficult.
With the feed circuit known, the length of the line can be determined from the DC current in the line. Then, the gain of amplifier circuits in the subscriber's terminal may be controlled to obtain a line-terminal combination having properties which are independent of the length of the line.
It is known, from German AS No. 22 54 148, to reduce the sending and receiving amplification of amplifier circuits in the subscriber's terminal, in linear proportion to the line DC current increase in excess of a predetermined minimum threshold value. Above a higher line DC current value, the two amplifications remain constant. The circuit of this references however, produces only a minimum threshold of the line DC current of about 1 mA which is too small since practical applications require 20 and 30 mA. Nothing can be learned from the reference about how to design a circuit for a higher minimum threshold current. Further, the gain variation with a linear drop over the line DC current obtained with this prior art circuitry does not correspond to the actual requirement, namely to obtain an amplification which is almost inversely proportional to the line DC current.